Corrosion resistant stent

ABSTRACT

A preferred embodiment is an uncoated, biodegradable stent. The stent has a filigree structure of magnesium alloy struts. The struts of the supporting structure are arranged to permit a compressed form for introduction into the body and to permit an expanded form at the site of the application within a vessel. The magnesium alloy struts are formed of a corrodible magnesium alloy. The magnesium alloy is formed from high-purity vacuum distilled magnesium containing impurities, which promote electrochemical potential differences and/or the formation of precipitations and/or intermetallic phases. The impurities are such that the struts of the stent have a tensile strength of &gt;275 MPa a yield point of &gt;200 MPa, a yield ratio of &lt;0.8, a difference between tensile strength and yield point of &gt;50 MPa.

PRIORITY CLAIM

This application is a continuation of and claims priority under 35 U.S.C. § 120 from prior pending U.S. application Ser. No. 14/395,709, which was filed on Oct. 20, 2014, which was a U.S. National Phase under 35 U.S.C. § 371 of International Application No. PCT/EP2013/063110, filed Jun. 24, 2013, which claims priority to U.S. Provisional Application No. 61/664,224, filed Jun. 26, 2012.

FIELD OF THE INVENTION

The invention concerns stents for the treatment of diseases. The stents provide a supporting function in hollow organs of a patient.

BACKGROUND

The implantation of stents has become established as one of the most effective therapeutic measures for the treatment of vascular diseases. Stents have the purpose of assuming a supporting function in hollow organs of a patient. For this purpose, stents featuring conventional designs have a filigree supporting structure comprising metal struts, which is initially present in compressed form for introduction into the body and is expanded at the site of the application. One of the main application areas of such stents is to permanently or temporarily widen and hold open vascular constrictions, particularly constrictions (stenosis) of coronary blood vessels. In addition, aneurysm stents are known, which are used primarily to seal the aneurysm. The support function is additionally provided by such aneurysm stents.

A stent has a base body made of an implant material. An implant material is a non-living material, which is employed for applications in medicine and interacts with biological systems. A basic prerequisite for the use of a material as an implant material, which is in contact with the body environment when used as intended, is biocompatibility. For the purpose of the present application, biocompatibility shall be understood to mean the ability of a material to induce an appropriate tissue reaction in a specific application. This includes an adaptation of the chemical, physical, biological, and morphological surface properties of an implant to the recipient's tissue with the aim of a clinically desired interaction. The biocompatibility of the implant material is also dependent on the temporal process of the reaction of the biosystem in which it is implanted. For example, irritations and inflammations occur in a relatively short time, which can lead to tissue changes. Depending on the properties of the implant material, biological systems thus react in different ways. According to the reaction of the biosystem, the implant materials can be divided into bioactive, bioinert and degradable or resorbable materials.

Conventional implant materials include polymers, metallic materials, and ceramic materials (as coatings, for example). Biocompatible metals and metal alloys for permanent implants include, for example, stainless steels (such as 316L), cobalt-based alloys (such as CoCrMo cast alloys, CoCrMo forge alloys, CoCrWNi forge alloys and CoCrNiMo forge alloys), pure titanium and titanium alloys (such as cp titanium, TiAl6V4 or TiAl6Nb7) and gold alloys. In the field of biocorrodible stents, the use of magnesium or pure iron as well as biocorrodible base alloys of the elements magnesium, iron, zinc, molybdenum, and tungsten have been proposed.

The use of biocorrodible magnesium alloys for temporary implants having filigree structures is made difficult in particular because degradation of the implant progresses very quickly in vivo. So as to reduce the corrosion rate, i.e., the degradation speed, different approaches are being discussed in the art. Modified alloys and coatings represent categories of approaches to reduce the corrosion rate of magnesium alloys. Some of the existing approaches show promise, but none of them has so far led to a commercially available product to the knowledge of the inventors. Regardless of the efforts made so far, there remains a need for solutions to at least temporarily reduce the corrosion of magnesium alloys in vivo, while optimizing the mechanical properties thereof at the same time.

Magnesium alloy properties are determined by the type and quantity of the alloying elements and impurities as well as the production conditions. The effects of the alloying elements and impurities on the properties of the magnesium alloys have been known to artisans. However, determining the properties of binary or ternary magnesium alloys for use as implant materials remains complex.

The alloying element used most frequently for magnesium is aluminum. Aluminum provides increased tensile strength due to solid solution and precipitation hardening and fine grain formation, but also in microporosity. Moreover, in the melt aluminum shifts the iron precipitation boundary toward drastically lower iron contents at which the iron particles precipitate or form intermetallic particles together with other elements.

Calcium exhibits a pronounced grain refining effect and worsens the castability and corrosion resistance.

Undesirable accompanying elements in magnesium alloys include iron, nickel, cobalt and copper, which cause a considerable increase in the corrosion tendency due to the electropositive nature thereof.

Manganese can be found in all magnesium casting alloys and binds iron in the form of AlMnFe precipitations, whereby the formation of local elements is reduced. On the other hand, manganese is not able to bind all the iron, and therefore a remainder of iron and a remainder of manganese are always left in the melt.

Silicon lowers the castability and viscosity, and as the content of Si rises, a worsened corrosion behavior is to be expected. Iron, manganese and silicon have a very high tendency to form an intermetallic phase. The electrochemical potential of this phase is very high and can thus act as a cathode controlling the corrosion of the alloy matrix.

As a result of solid solution hardening, zinc improves the mechanical properties and results in grain refining, however it also leads to microporosity with a tendency toward hot cracking starting at a content of 1.5 to 2% by weight in binary Mg—Zn and ternary Mg—Al—Zn alloys.

Alloying additions made of zirconium increase the tensile strength without lowering the expansion and lead to grain refining, but also to a strong impairment of dynamic recrystallization, which is manifested in an increase of the recrystallization temperature and therefore requires high energy expenditure. Moreover, zirconium cannot be added to melts containing aluminum and silicon because the grain refining effect is lost.

Rare earths such as Lu, Er, Ho, Th, Sc and In all exhibit a similar chemical behavior and form eutectic systems with partial solubility on the magnesium-rich side of the binary phase diagrams such that precipitation hardening is possible.

The addition of further alloying elements, in conjunction with the impurities, is known to cause the formation of different intermetallic phases in binary magnesium alloys. For example, the intermetallic phase Mg₁₇Al₁₂ forming at the grain boundaries is brittle and limits the ductility. As compared to the magnesium matrix, this intermetallic phase is more noble and able to form local elements, whereby the corrosion behavior worsens.

In addition to these influencing factors, the properties of the magnesium alloys also decisively depend on the metallurgical production conditions. Conventional casting methods automatically introduce impurities when adding, by alloying, the alloying elements. The prior art (U.S. Pat. No. 5,055,254 A) therefore defines tolerance limits for impurities in magnesium casting alloys, which, for example for a magnesium-aluminum-zinc alloy containing approximately 8 to 9.5% by weight Al and 0.45 to 0.9% by weight Zn, mentions tolerance limits of 0.0015 to 0.0024% by weight Fe, 0.0010% by weight Ni, 0.0010 to 0.0024% by weight Cu and no less than 0.15 to 0.5% by weight Mn.

Tolerance limits for impurities in magnesium and the alloys thereof as well as the production conditions are mentioned in many known documents and listed as follows in % by weight:

Alloy Production State Fe Fe/Mn Ni Cu Pure Mg no information 0.017 0.005 0.01 AZ 91 Die casting F 0.032 0.005 0.040 High-pressure 0.032 0.005 0.040 die casting Low-pressure 0.032 0.001 0.040 die casting T4 0.035 0.001 0.010 T6 0.046 0.001 0.040 Gravity die F 0.032 0.001 0.040 casting AM60 Die casting F 0.021 0.003 0.010 AM50 Die casting F 0.015 0.003 0.010 AS41 Die casting F 0.010 0.004 0.020 AE42 Die casting F 0.020 0.020 0.100

It has been found that these tolerance definitions are not sufficient to reliably exclude the formation of corrosion-promoting intermetallic phases, which in terms of electrochemistry have a more noble potential than the magnesium matrix.

Biodegradable implants, including stents, require a load-bearing function and consequently strength, together with sufficient expandability, during the physiologically necessary support periods thereof. Known magnesium materials fall far short of the strength properties provided by permanent implants made from other materials such as titanium, CoCr alloys and titanium alloys. The ultimate tensile strength R_(m) for permanent implants is approximately 500 MPa to >1000 MPa, while that of magnesium materials is <275 MPa so far, and in most cases <250 MPa.

Another drawback of many prior magnesium materials for use in stents is that the difference between ultimate tensile strength R_(m) and proof stress R_(p) is small. In the case of implants that allow plastic deformation, such as cardiovascular stents, this means that no further resistance exists against deformation after initial deformation of the material, and the regions that have already been deformed are deformed further without any load increase. This can lead to overstretching of parts of the component and fracture may occur.

Many magnesium materials additionally exhibit a clearly pronounced mechanical asymmetry, which is manifested in the difference in the mechanical properties, especially the proof stress R_(p) with tension load and compression load. Such asymmetries are created, for example, during forming processes such as extrusion, rolling and drawing, which are used to produce suitable semi-finished products. A difference between the proof stress R_(p) during tension and the proof stress R_(p) during compression that is too large may result in inhomogeneous deformation of a component, such as a cardiovascular stent, which later undergoes multiaxial deformation, and may cause cracking and fracture.

Because of the low number of crystallographic slip systems, magnesium alloys can generally also form textures during forming processes such as extrusion, rolling and drawing used to produce suitable semifinished products by orienting the grains during the forming process. Specifically, this means that the semifinished product has different properties in different directions in space. For example, high deformability or elongation at fracture occurs in one direction in space after forming, and reduced deformability or elongation at fracture occurs in another direction in space. The formation of such textures should likewise be avoided, because a stent is subjected to high plastic deformation, and reduced elongation at fracture increases the risk of failure of the implant. One method for substantially avoiding such textures during forming is to adjust as fine a grain as possible prior to forming. Because of the hexagonal lattice structure of magnesium materials, the ability of these materials to deform at room temperature is low, which is characterized by slip in the base plane. If the material additionally has a coarse microstructure, i.e., a coarse grain, so-called twinning is forcibly produced upon further deformation, at which shear strain occurs, which transforms a crystal region into a position that is mirror symmetrical to the starting position. The resulting twin grain boundaries constitute weak points in the material, where incipient cracking starts, especially with plastic deformation, which ultimately leads to the destruction of the component.

If the grain of the implant materials is sufficiently fine, the risk of such implant failure is drastically reduced. Implant materials should therefore have as fine a grain as possible so as to prevent such undesirable shear strain.

All available magnesium materials for implants are subject to high corrosion in physiological media. Attempts have been made to curb the corrosion tendency by providing the implants with a corrosion-inhibiting coating, for example made of polymeric materials (EP 2 085 100 A2, EP 2 384 725 A1), an aqueous or alcoholic conversion solution (DE 10 2006 060 501 A1) or an oxide (DE 10 2010 027 532 A1, EP 0 295 397 A1).

The polymeric passivation layers are controversial, because virtually all appropriate polymers also cause strong inflammations in the tissue at times. On the other hand, thin magnesium alloy structures without such protective measures do not resist corrosions for the required support periods. The corrosion on thin-walled traumatological implants is often times accompanied by an excessively fast loss of tensile strength, which poses an additional burden by forming excessive amounts of hydrogen per unit of time. The consequences are undesirable gas inclusions in the bones and tissue. In the case of traumatological implants having larger cross-sections, there is a need to be able to deliberately control the hydrogen problem and the corrosion rate of the implant by way of the structure thereof.

Specifically with biodegradable implants, there is a desire for maximum biocompatibility of the elements, because all the chemical elements that are contained are absorbed by the body after decomposition. In any case, highly toxic elements such as Be, Cd, Pb, Cr and the like should be avoided.

Degradable magnesium alloys are especially suitable for implementing implants. Stents are used, for example, to support vessels, hollow organs and vein systems (endovascular implants, such as stents).

SUMMARY OF THE INVENTION

A preferred embodiment is a biodegradable stent. The stent has a filigree structure of magnesium alloy struts. The struts of the supporting structure are arranged to permit a compressed form for introduction into the body and to permit an expanded form at the site of the application within a vessel. The magnesium alloy struts are formed of a corrodible magnesium alloy having less or equal 4.0% by weight Zn, 2.0 to 10.0% by weight Al, the alloy content of Al in % by weight being greater than or equal to the alloy content of Zn in % by weight defining an alloy matrix having solid solutions of Al and Zn and intermetallic phases of magnesium and Al in the alloy matrix, the matrix lacking Mn as an alloying element to suppress formation of the ternary intermetallic phase FeMnSi and thereby improve corrosion resistance, the remainder being a, in a total amount of no more than 0.0063% by weight of Fe, Si, Mn, Co, Ni, Cu, Zr, Y, Sc, rare earth lanthanoids and actinoids, Be, Cd, In, Sn and/or Pb as well as P, wherein the matrix of the alloy is solid solution hardening due to Al and Zn and is also particle hardening due to the intermetallic phases formed of Mg and Al, wherein the struts of the stent have a tensile strength of >275 MPa a yield point of >200 MPa, a yield ratio of <0.8, a difference between tensile strength and yield point of >50 MPa.

A preferred embodiment is a biodegradable stent. The stent has a filigree structure of magnesium alloy struts. The struts of the supporting structure are arranged to permit a compressed form for introduction into the body and to permit an expanded form at the site of the application within a vessel. The magnesium alloy struts are formed of a corrodible magnesium alloy having 2.0 to 10.0% by weight Al, the alloy having solid solutions of Al and intermetallic phases of magnesium and Al in the alloy matrix, the matrix lacking Mn as an alloying element to suppress formation of the ternary intermetallic phase FeMnSi and thereby improve corrosion resistance, the remainder being high-purity vacuum distilled magnesium containing impurities, which promote electrochemical potential differences and/or the formation of precipitations and/or intermetallic phases, in a total amount of no more than 0.0063% by weight, wherein the matrix of the alloy is solid solution hardening due to Al and is also particle hardening due to the intermetallic phases formed of Mg and Al, wherein the struts of the stent have a tensile strength of >275 MPa a yield point of >200 MPa, a yield ratio of <0.8, a difference between tensile strength and yield point of >50 MPa.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment provides a biodegradable stent. The stent has a filigree supporting structure of magnesium alloy struts. The supporting structure permits a compressed form for introduction into the body. The supporting structure can be expanded (e.g., through use of a balloon or through self-expansion) at the site of the application within a vessel. The magnesium alloy of the struts has very high corrosion resistance, which is achieved by drastically reducing the content of impurities and the combinations thereof in the magnesium matrix, and by having only those alloying elements present which raise the electrochemical potential of the matrix such that the corrosion resistance of the alloy is considerably increased. Corrosion resistance and deformability of the magnesium matrix of an stent with an alloy of the invention can be assured over a support period such that the stent is able to absorb multiaxial permanent load without fracture or cracking, and the stent can also benefit from the magnesium matrix for decomposition triggered by the physiological liquids.

The previously known tolerance limits for impurities do not take into account that wrought magnesium alloys often times are subjected to a thermomechanical treatment, and more particularly to an extended annealing process, which creates the near-equilibrium structures. The metallic elements bond by way of diffusion and form what are known as intermetallic phases, which have a different electrochemical potential, notably a considerably higher potential, than the magnesium matrix, and therefore these intermetallic phases act as cathodes and can trigger galvanic corrosion processes.

The applicant has found that a corrosion-stable alloy matrix can be achieved when complying with the following tolerance limits of individual impurities in % by weight: Fe, Si, Mn, Co, Ni, Cu each with <0.0005; Zr, Y each with <0.0003; and P<0.0002. Further preferred in this embodiment, the alloy has additional individual impurities in the following tolerance limits (% by weight): Sc or rare earths having the ordinal numbers 21, 57 to 71 and 89 to 103 in total <0.001; Be, Cd, In, Sn and/or Pb each with <0.0003.

Preferably, the corrosion-staple alloy matrix contains impurities in a total amount of no more than 0.0053 Gew. %, which can be achieved when complying with the following tolerance limits of individual impurities in % by weight: Fe, Si, Mn each with <0.0005; Co, Ni, Cu each with <0.0002; Zr, Y each with <0.0003; Sc or rare earths having the ordinal numbers 21, 57 to 71 and 89 to 103 in total <0.001; Be, Cd, In, Sn and/or Pb each with <0.0003; and P<0.0001.

In particular preferred embodiments, the corrosion-staple alloy matrix contains impurities in a total amount of no more than 0.0022 Gew. %, which can be achieved when complying with the following tolerance limits of individual impurities in % by weight: n: Fe, Si, Mn each with <0.0002; Co, Ni, Cu, Zr, Y each with <0.0001; Sc or rare earths having the ordinal numbers 21, 57 to 71 and 89 to 103 in total <0.0005; Be, Cd, In, Sn and/or Pb each with <0.0001, and P<0.0001.

It is surprising that the addition of manganese as an alloying element, which is customary in the prior art, can be dispensed with when these tolerance limits are adhered. The formation of the ternary intermetallic phase FeMnSi is suppressed, whereby the corrosion resistance of the alloy is improved.

When the impurity elements are combined, this tolerance limit of the sum of impurities of Fe, Si, Mn, Co, Ni and Cu is no more than 0.003% by weight, preferably no more than 0.0021% by weight and in particular preferred no m ore than 0.0009% by weight.

Preferred magnesium alloys according to the present invention can achieve a tensile strength of >275 MPa, and preferably >300 MPa, a yield point of >200 MPa, and preferably >225 MPa, and a yield ratio of <0.8, and preferably <075, wherein the difference between the tensile strength and yield point is >50 MPa, and preferably >100 MPa.

These significantly improved mechanical properties can be attributed to the formation of solid solutions between aluminum and zinc in the alloy matrix as well as to the intermetallic phases of magnesium and aluminum occurring in the matrix. The basis for the increased tensile strength is the interaction of the dislocations with the particles, whereby the dislocation movement is adversely impacted and additional tension is required in order to generate the same plastic deformation as in an undisturbed matrix.

The improved mechanical properties of the novel magnesium alloys assure that the stents, for example cardiovascular stents, are able to withstand the multiaxial permanent load in the implanted state over the entire support period, despite onsetting degradation of the magnesium matrix due to corrosion.

For the mechanical asymmetry, it is particularly important for the magnesium alloy to have a particularly fine microstructure having a grain size of no more than 7.5 μm, preferably <5 μm, and still more preferably <2.5 μm.

A preferred method for producing a magnesium alloy having the improved mechanical and electrochemical properties is also provided by the invention. The method comprises the following steps:

-   a) generating high-purity magnesium by vacuum distillation; -   b) generating a billet of the alloy by synthesis of the magnesium     according to step a) with less or equal 4.0% by weight Zn, 2.0 to     10.0% by weight Al, wherein the alloy content of Al in % by weight     is greater than or equal to the alloy content of Zn in % by weight,     the remainder being magnesium containing impurities, which promote     electrochemical potential differences and/or the formation of     precipitations and/or intermetallic phases, in a total amount of no     more than 0.0063% by weight of Fe, Si, Mn, Co, Ni, Cu, Zr, Y, Sc or     rare earths having the ordinal numbers 21, 57 to 71 and 89 to 103,     Be, Cd, In, Sn and/or Pb as well as P, wherein the matrix of the     alloy is solid solution hardening due to Al and Zn and is also     particle hardening due to the intermetallic phases formed of Mg and     Al; -   c) homogenizing the alloy by annealing at a temperature between     150° C. and 450° C., with a holding period of 4 to 40 hours; and -   d) forming the homogenized alloy in the temperature range between     200° C. and 400° C.

Preferably steps c) and d) can be repeated at least once.

After step c) and before step d) an ageing treatment step can be performed. Depending on the alloy composition and/or the amount and/or type of the grains and/or the grain size ageing treatment step can be performed at a temperature between 20° C. and 300° C. with a holding period of 1 h to 168 h. Preferably the ageing treatment can be performed at a temperature between 20° C. and 275° C., still more preferably at a temperature of 150° C. with a holding period of 120 hours.

In a preferred value range steps c) is performed at a temperature between 250° C. and 450° C. and/or step d) is performed at a temperature between 225° C. and 400° C.

Preferably, the magnesium alloy generated by step a) has a content of Zn less or equal 2.0% by weight, in particular preferably less or equal 1.0% by weight and/or a content of Al in the range of 2.0 to 8.0% by weight, preferably 3.0 to 8.0% by weight and still more preferably 3.0 to 6.0% by weight.

Vacuum distillation is preferably used to produce a starting material for a high-purity magnesium-aluminum-zinc alloy having the required threshold values.

The sum of impurities can be selectively adjusted and in % by weight are:

-   a) for the individual impurities:     -   Fe, Si, Mn, Co, Ni, Cu each with <0.0005;     -   Zr, Y each with <0.0003; and     -   P<0.0002.     -   Preferably in this embodiment additional individual impurities         in the following tolerance limits (% by weight):     -   Sc or rare earths having the ordinal numbers 21, 57 to 71 and 89         to 103 in total <0.001; Be, Cd, In, Sn and/or Pb each with         <0.0003.     -   aa) for the individual impurities in a preferred total amount of         impurities of no more than 0.0053% by weight:         -   Fe, Si, Mn each with <0.0005;         -   Co, Ni, Cu each with <0.0002;         -   Zr, Y each with <0.0003;         -   Sc or rare earths having the ordinal numbers 21, 57 to 71             and 89 to 103 in total <0.001;         -   Be, Cd, In, Sn and/or Pb each with <0.0003; and         -   P<0.0001.     -   ab) for the individual impurities in a particularly preferred         total amount of impurities of no more than 0.0022% by weight:         -   Fe, Si, Mn each with <0.0002;         -   Co, Ni, Cu, Zr, Y each with <0.0001;         -   Sc or rare earths having the ordinal numbers 21, 57 to 71             and 89 to 103 in total <0.0005;         -   Be, Cd, In, Sn and/or Pb each with <0.0001; and         -   P<0.0001. -   b) for the combination of individual impurities in total:     -   Fe, Si, Mn, Co, Ni, Cu no more than 0.003, preferably no more         than 0.0021% by weight and in particular preferred no m ore than         0.0009% by weight.

It is particularly advantageous that the preferred method only requires a small number of forming steps. Extrusion, equal channel angular extrusion and/or multiple forging can thus preferably be employed, which assure that a substantially homogeneous fine grain of <10 μm is achieved.

The magnesium alloy produced according to the method, which has the above described advantageous composition and structure, in medical technology, can also be used in the production of implants, for example endovascular implants such as stents, for fastening and temporarily fixing tissue implants and tissue transplantations, orthopedic and dental implants, and neuroimplants.

Particular implants of the invention are in the Cardiovascular field, osteosynthesis field or other areas.

Cardiovascular field in the sense of this application includes

-   -   the field of diagnostic, prevention and treatment of all         diseases of the cardiovascular system, i.e. heart and blood         vessel system,     -   by mean of active and non-active implants used to support         vessels, and vein systems     -   including coronary, cerebral and peripheral vascular implants         like stents, valves, closure devices, occluders, clips, coils,         staples, implantable regional drug delivery devices,     -   implantable electrostimulators (like pacemakers and         defibrillators), implantable monitoring devices, implantable         electrodes,     -   system for fastening and temporarily fixing tissue implants and         tissue transplantations     -   field also includes any type of stent as mechanical fix or         temporary scaffold to support hollow organs and structures         including bones, intervertebral disks

Osteosynthesis in the sense of this application includes

-   -   the field of treatment of fractured bones for internal fixation         and stabilization by mechanical devices such as metal plates,         pins, rods, wires, screws, clips, nails, staples excluding stent         technology

Examples of areas out of the osteosynthesis field or the cardiovascular field are:

-   -   Devices for the treatment of diseases of the sinews, joints,         muscles, cartilages,     -   oral (including dental) and maxillo facial implants (excl.         osteosynthesis means),     -   esthetic implants,     -   supporting tools out of the body,     -   tissue engineering,     -   soft tissue implants,     -   devices for wound care,     -   suture material and clamps,     -   neurosurgery     -   local drug delivery (excl. cardiovascular, i.e. lever)     -   renal

Exemplary Embodiment 1

A magnesium alloy is generated which is composed of 2.0% by weight Zn and 6.0% by weight Al, the remainder being Mg, and contains the following individual impurities in % by weight:

Fe: <0.0005; Si: <0.0005; Mn: <0.0005; Co: <0.0002; Ni: <0.0002; Cu <0.0002, wherein the sum of impurities of Fe, Si, Mn, Co, Ni, Cu is no more than 0.0021% by weight and that of Zr is no more than 0.0003% by weight.

The magnesium produced with aid of vacuum distillation is melted with high-purity Al and Zn in a graphite crucible, and the alloy is subjected to homogenizing annealing at a temperature of 360° C. for a duration of 24 hours, and subsequently to multiple extrusion processes at a temperature of 300° C., so as to produce a precision tube for a cardiovascular stent.

The grain size of the microstructure was <5.5 μm, and the particle size of the intermetallic phases dispersely distributed in the alloy matrix was 0.5 μm.

The magnesium alloy reached a tensile strength of 310 to 320 MPa and proof stress of approximately 250 MPa [sic]. The yield ratio was 0.79 and the mechanical asymmetry was 1.2.

Exemplary Embodiment 2

A magnesium alloy is generated which is composed of 0.25% by weight Zn and 2.50% by weight Al, the remainder being Mg, and contains the following individual impurities in % by weight:

Fe: <0.0005; Si: <0.0005; Mn: <0.0005; Co: <0.0002; Ni: <0.0002; Cu <0.0002, wherein the sum of impurities of Fe, Si, Mn, Co, Ni, Cu is no more than 0.0021% by weight and that of Zr is no more than 0.0003% by weight.

The magnesium produced with aid of vacuum distillation is melted with high-purity Al and Zn in a graphite crucible, and the alloy is subjected to homogenizing annealing at a temperature of 360° C. for a duration of 24 hours, and subsequently to multiple extrusion processes at a temperature of 300° C., so as to produce a precision tube for a cardiovascular stent.

The grain size of the microstructure was <5.5 μm, and the particle size of the intermetallic phases dispersely distributed in the alloy matrix was 0.5 μm.

The magnesium alloy reached a tensile strength of 310 to 320 MPa and proof stress of approximately 250 MPa [sic]. The yield ratio was 0.79 and the mechanical asymmetry was 1.2.

Exemplary Embodiment 3

A magnesium alloy is generated which is composed of 5.0% by weight Al, the remainder being Mg, and contains the following individual impurities in % by weight:

Fe: <0.0005; Si: <0.0005; Mn: <0.0005; Co: <0.0002; Ni: <0.0002; Cu <0.0002, wherein the sum of impurities of Fe, Si, Mn, Co, Ni, Cu is no more than 0.0021% by weight and that of Zr is no more than 0.0003% by weight.

The magnesium produced with aid of vacuum distillation is melted with high-purity Al in a graphite crucible, and the alloy is subjected to homogenizing annealing at a temperature of 360° C. for a duration of 24 hours, and subsequently to multiple extrusion processes at a temperature of 300° C., so as to produce a precision tube for a cardiovascular stent.

The grain size of the microstructure was <5.5 μm, and the particle size of the intermetallic phases dispersely distributed in the alloy matrix was 0.5 μm.

The magnesium alloy reached a tensile strength of 310 to 320 MPa and proof stress of approximately 250 MPa [sic]. The yield ratio was 0.79 and the mechanical asymmetry was 1.2.

Exemplary Embodiment 4

A magnesium alloy is generated which is composed of 3% by weight Al, the remainder being Mg, and contains the following individual impurities in % by weight:

Fe: <0.0005; Si: <0.0005; Mn: <0.0005; Co: <0.0002; Ni: <0.0002; Cu <0.0002, wherein the sum of impurities of Fe, Si, Mn, Co, Ni, Cu is no more than 0.0021% by weight and that of Zr is no more than 0.0003% by weight.

The magnesium produced with aid of vacuum distillation is melted with high-purity Al in a graphite crucible, and the alloy is subjected to homogenizing annealing at a temperature of 360° C. for a duration of 24 hours, and subsequently to multiple extrusion processes at a temperature of 300° C., so as to produce a precision tube for a cardiovascular stent.

The grain size of the microstructure was <5.5 μm, and the particle size of the intermetallic phases dispersely distributed in the alloy matrix was 0.5 μm.

The magnesium alloy reached a tensile strength of 310 to 320 MPa and proof stress of approximately 250 MPa [sic]. The yield ratio was 0.79 and the mechanical asymmetry was 1.2.

Exemplary Embodiment 5

A magnesium alloy is generated which is composed of 0.25% by weight Zn and 2.0% by weight Al, the remainder being Mg, and contains the following individual impurities in % by weight:

Fe: <0.0005; Si: <0.0005; Mn: <0.0005; Co: <0.0002; Ni: <0.0002; Cu <0.0002, wherein the sum of impurities of Fe, Si, Mn, Co, Ni, Cu is no more than 0.0021% by weight and that of Zr is no more than 0.0003% by weight.

The magnesium produced with aid of vacuum distillation is melted with high-purity Al and Zn in a graphite crucible, and the alloy is subjected to homogenizing annealing at a temperature of 360° C. for a duration of 24 hours and thereafter to an ageing treatment at 125° C. for 120 hours.

Subsequently, the material is subjected to multiple extrusion processes at a temperature of 200° C., so as to produce a precision tube for a cardiovascular stent.

Before the final extrusion step is applied another annealing process is performed at 150° C. for 3 hours.

The grain size of the microstructure was <5.5 μm, and the particle size of the intermetallic phases dispersely distributed in the alloy matrix was 0.5 μm.

The magnesium alloy reached a tensile strength of 320 to 350 MPa and proof stress of approximately 235 MPa. The yield ratio was 0.70 and the mechanical asymmetry was 1.2.

Exemplary Embodiment 6

A magnesium alloy is generated which is composed of 1.5% by weight Zn and 3.0% by weight Al, the remainder being Mg, and contains the following individual impurities in % by weight:

Fe: <0.0005; Si: <0.0005; Mn: <0.0005; Co: <0.0002; Ni: <0.0002; Cu <0.0002, wherein the sum of impurities of Fe, Si, Mn, Co, Ni, Cu is no more than 0.0021% by weight and that of Zr is no more than 0.0003% by weight.

The magnesium produced with aid of vacuum distillation is melted with high-purity Al and Zn in a graphite crucible, and the alloy is subjected to homogenizing annealing at a temperature of 360° C. for a duration of 24 hours and thereafter to an ageing treatment at 150° C. for 120 hours.

Subsequently, the material is subjected to an extrusion process at a temperature of 200° C., so as to produce a rod with 8 mm diameter to produce screws for craniofacial fixations.

The grain size of the microstructure was <3.0 μm, and the particle size of the intermetallic phases dispersely distributed in the alloy matrix was 0.5 μm.

The magnesium alloy reached a tensile strength of 340 to 360 MPa and proof stress of approximately 250 MPa [sic]. The yield ratio was 0.71 and the mechanical asymmetry was 1.2.

While specific embodiments of the present invention have been shown and described, it should be understood that other modifications, substitutions and alternatives are apparent to one of ordinary skill in the art. Such modifications, substitutions and alternatives can be made without departing from the spirit and scope of the invention, which should be determined from the appended claims.

Various features of the invention are set forth in the appended claims. 

The invention claimed is:
 1. A biodegradable stent, comprising a filigree structure of magnesium alloy struts, the struts of the filigree structure being arranged to permit a compressed form for introduction into the body and to permit an expanded form at the site of the application within a vessel; wherein the magnesium alloy struts consist of a corrodible magnesium alloy having 2.0 to 10.0% by weight Al and a remainder consisting of vacuum distilled magnesium containing impurities in a total amount of no more than 0.0063% by weight, the alloy having solid solutions of Al and intermetallic phases of magnesium and Al in the alloy matrix, the matrix lacking Mn as an alloying element to suppress formation of the ternary intermetallic phase FeMnSi and thereby improve corrosion resistance, wherein the struts of the stent have a tensile strength of >275 MPa a yield point of >200 MPa, a yield ratio of <0.8, and a difference between tensile strength and yield point of >50 MPa.
 2. The biodegradable stent of claim 1, wherein the magnesium alloy struts have a microstructure with a grain size of <5.5 μm.
 3. The biodegradable stent of claim 2, wherein the intermetallic phases are dispersedly distributed and have a 0.5 μm particle size.
 4. The biodegradable stent of claim 1, wherein the content of Al is 2.0 to 8.0% by weight.
 5. The biodegradable stent of claim 1, wherein the impurities total no more than 0.003% by weight.
 6. The biodegradable stent of claim 1, wherein the magnesium alloy struts have a fine-grained microstructure having a grain size of no more than 7.5 μm.
 7. The biodegradable stent of claim 1, wherein the magnesium alloy struts are uncoated by any polymer passivation layer or any metal or ceramic corrosion-inhibiting coating.
 8. The biodegradable stent of claim 7, wherein the stent is an active stent. 